This invention relates to a TV tuner circuit having a dual-gate MES (Metal Semiconductor) FET in a UHF RF amplifier and a dual-gate MOS FET in a VHF RF amplifier which are both controlled by a single AGC voltage source. Though not an object of this invention, the TV tuner circuit using a dual-gate MES FET in UHF RF amplifier improves noise performance and cross modulation performance of the tuner.
In a prior art, a dual-gate MES FET in a UHF RF amplifier and a dual-gate MOS FET in a VHF RF amplifier are introduced into a conventional tuner circuit.
An AGC voltage is always applied to both the second gate of the dual-gate MES FET and that of the dual-gate MOS FET. In receiving UHF signals, DC bias voltages are applied to the source, the first gate and the drain of the dual-gate MES FET through resistors from a power supply source. But no DC bias voltage are applied to the dual-gate MOS FET in the VHF amplifier. In receiving VHF signals, DC bias voltages are applied to the source, the first gate and the drain of the dual-gate MOS FET through resistors from the power supply source. But no DC bias voltages are applied to the source, the first gate and the drain of the dual-gate MES FET.
Therefore, in this prior art tuner, the voltage between the source and the second gate of the dual-gate MES FET becomes so high that a DC current is obliged to flow from the AGC voltage source in the VIF circuit to the second gate of the dual-gate MES FET in receiving VHF signals.
Due to the internal resistance of the AGC voltage source, the AGC voltage applied to the second gate of the dual-gate MOS FET in the VHF RF amplifier drops in comparison with that of the dual-gate MES FET in the UHF RF amplifier (with the same input signal strength of UHF and VHF signals).
The input signal strength at which the gain reduction of the VHF RF amplifier starts will not be the same as that at which that of the UHF RF amplifier starts.